Use of AFM to probe the adsorption strength and time‐dependent changes of albumin on self‐assembled monolayers

Abstract

The adsorption kinetics of human serum albumin (HSA) on CH₃‐ and COOH‐terminated self‐assembled monolayers (SAMs) has been investigated using radioassays and atomic force microscopy (AFM). On both surfaces, the amount of HSA adsorbed reached a plateau after 30 min. The plateau level was higher on the CH₃ compared to the COOH surface. The adhesion force (F_adh), measured using Si₃N₄ AFM tips in water, decreased with time of contact with the HSA solution on the CH₃ surface. This time‐dependent change in the adhesiveness of the adsorbed protein is best explained by a change in the conformation or orientation. In contrast, F_adh was independent of the time of contact with the HSA solution on the COOH surface, indicating that once adsorbed, the HSA molecules do not undergo further conformation or orientation changes. The perturbation induced by scanning with the AFM in water on the adsorbed HSA layers was greater on CH₃ surfaces than on COOH surfaces, suggesting a weaker protein–substratum interaction on the CH₃‐terminated SAMs. This was further confirmed by a stronger desorption of HSA following sodium dodecyl sulfate (SDS) treatment on the CH₃ surface compared to the COOH surface. Taken together, these data suggest that for COOH SAMs, (1) there is a strong interaction between HSA and the substratum; (2) there is an absence of reorientation with time; and (3) there is a smaller amount of adsorbed protein at 24 h, possibly due to increased but rapid spreading/denaturation of the protein. On the CH₃ surface, less deformation of HSA occurs and the molecules maintain a higher mobility at short adsorption times. AFM measurements performed after aging of an adsorbed HSA layer in buffer suggests the role played by HSA in solution in determining the time‐dependent conformation and/or orientation changes. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res 67A: 548–558, 2003